Automatic frequency control system



Patented July 16, 1940 2,207,934

UNITED STATESPATENT OFFICE AUTOMATIC FREQUENCY CONTROL SYSTEM 4 Garrard Mountjoy, Manhasset, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 12, 1938, Serial No. 201,500

8 Claims. (01. 25020) My present invention relates to receivers of the provided with a. tunable tank circuit 6. The dottype employing automatic frequency control ted lines denoted Tuner show the mechanical (AFC), and more particularly to such a receiver uni-control coupling between the rotors of the embodying a novel control mechanism for the variable tuning condensers of circuits I3-6.

,6 AFC. The local oscillator tank circuit 6 is tunable over Receivers of the AFC type often utilize a plua frequency range which differs at all times from rality of cascaded resonant circuits to feed the the frequency range of the signal selector ciraudio detector, While the discriminator rectifiers cults I and 3 by the value of the intermediate are connected to an intermediate one of. the casfrequency; say, for example, 400 kc.

caded circuits. When such a receiver is of the The I. F. energy is transmitted through I. F. 10

multi-range type as well, and AFC action is deamplifiers 'I--"I'; each of the latter is fixedly ressired only on less than all the wave bands, it is onated to the operating I. F. The I. F.-tuned necessary to use a mechanism for eliminating the output circuit 8 of the I. F. amplifier I is cou- AFC action over the frequency range where it pled to a discriminator input circuit 9; the syml is not desired. Mere opening, or short-circuiting, bol M designates the magnetic coupling between of the AFC lead to the frequency control tube the circuits 8 and 9. The construction of the is not adequate; such expedient in no way imdiscriminator follows the disclosure of the aforeproves the reception characteristic. For examsaid Foster patent. v Briefly, the network comple, an improvement in the selectivity characterprises diodes II] and II; load resistors I2 and I3 istic is desirable when the has been elimbeing connected in series between the cathodes 20 mated. I of the diodes, and the cathode of diode II being It may be stated that it is one of the main grounded. The I. F. by-pass condenser I4 is conobjects of my present invention to provide in an nected across the load resistors; the junction I5 AFC receiver of the type employing cascaded of the latter is connected by lead 2| to the mid- :86 resonant discriminator and audio demodulator point I6 of the secondary coil 9'. The direct circuits, a device for substantially reducing the current blocking condenser ll connects the high impedance loading of the discriminator resonant potential side of the primary circuit 8 to midinput circuit when the AFC action is rendered point I6.

,inefiective over a desired frequency range there- The switch mechanism 20 is disposed in the Q0 by substantially to improve the selectivity to the lead 2|. When the switch 20 is open, the disaudio demodulator. criminator is ineffective to produce AFC bias.

Another important object of my invention may The audio demodulator comprises the input cirbe stated to reside in the provision of a multicult. 30 arranged in series with diode SI and load band receiver provided with AFC; means being resistor 32. The circuit 30 is magnetically couprovided to act jointly with the wave band changpled, as at M1, to circuit 9. k The cathode of diode 85 ing mechanism to render the AFC ineffective over 3| is at ground potential, while load resistor 32 is at least one of the reception bands and simulby-passed for I. F. current by condenser 33. The taneously to improve the selectivity to the audio direct current voltage component of detected I. F. demodulator. current is used for AVC' action. The line 34, in-

45 Still other objects of the invention are to eluding a filter 35, acts as the AVG connection to 40 improve generally the operation of AFC receivers. the I. F. amplifier, and to any of the prior tubes of the multi-band type, and more especially to when desired. The audio voltage across load provide such receivers in an economical manner. resistor 32 is transmitted, by path 40, to a de- Referring now to the accompanying drawsired audio network.

.a ing, there is shown therein a well known type The discriminator functions, as is now well of superheterodyne receiver employing AFC. For known by those skilled in the art, to provide example, the receiver may be of the type disacross resistors I2 and I3 in series, a direct ourclosed, and claimed, by D. E. Foster in applicarent voltage (AFC bias) whose polarity and magtion Serial No. 72,495, filed April 3, 1936, patented nitude are dependent upon the direction and =69 June 2:1, 1938, as U. S. P. 2,120,974. Such a reamount 'of frequency shift, respectively, of the .50 celver comprises a signal collector A feeding the I. F. carrier from the assigned 1. F. value. Each tunable selector circuit I of a radio frequency .amof circuits 8 and 9 is tuned to the assigned I. F. plifier 2; the amplified modulated carrier signals value; as is, also, the resonant circuit 30. As is are impressed on the tunable selector circuit 3 explained fully in the aforesaid Foster patent,

.of the .first detector 4. The local oscillator 5 is the selectivity at the audio detector 3I is high as I. F. value.

by virtue of the fact that circuits 8930 are in cascade. Instead of taking audio voltage from point 15, extra selectivity is secured by adding the network 303l.

The AFC bias, developed between the cathode end of resistor l2 and ground, is impressed upon the frequency control tube 50 by means of the.

line 5|. The latter is designated by the symbol AFC; and includes a filter 52 for suppressing pulsation components in the AFC bias. The tube 50 may have its circuits arranged in any manner Well known to those skilled in the art; the function of the tube is to simulate across tank circuit 6 a reactance, and the latter may be positive or negative in sign. Reference is made to the application of C. Travis, Serial No. 19,563, filed May 3, 1935, for various types of circuits which may be used at 50. The AFC bias varies the gain of tube 50 in a sense such that the simulated reactance, across tank 6, adjusts the latter in frequency to maintain the assigned I. F. value at the output of network 4. It is not believed necessary to explain the operation of the system up to this point in any further detail. The present invention is independent of the particular construction of the discriminator; 'what is important is that the functioning of the discriminator can be prevented with improved selectivity to the detector 3| secured.

Without going into a detailed explanation, then, of the AFC system, and reference bein made again for that purpose to the said Foster patent, it will be understood that as the receiver is tuned by the tuner the AFC acts to adjust the oscillator tank 6 so as to securethe correct For example, if the tuning mechanism has been adjusted to receive a desired carrier, but is not accurately set thereto, the local oscillations will beat with the desired signals to produce I. F. energy whose carrier differs from the resonant frequency of circuits 8-9. The carrier shift is translated into a direct current voltage across l 2l3; when the carrier is equal to the assigned I. F. value, then the drop across l2-l3 is zero. It is assumed that the switch 2!] is closed; this permits each of the rectifiers l0 and H to rectify the sum of the voltage across circuit 8 and the voltage induced across one half of the secondary coil 9. The phase relation between the voltages induced across the two halves of coil 9' depends upon the frequency of the applied I. F. energy. Hence, it will be seen that the diodes I 0 and I l and load resistors [2, I3, comprise a load upon the input circuit 9.

The AFC bias across resistors I2 and l3 is used to vary the gain of tube 50. The reactance simulated by tube 50 is, therefore, varied; the variation is such as to adjust. the frequency of tank circuit 6 to a value which finally results in I. F. energy of the assigned carrier value. The lead 5| could be opened, or short-circuited, to render ineffective the AFC action. However, the loading of the input circuit 9 would still exist, and hence the selectivity to the detector 3| would be impaired to that extent.

By providing the switch 20 in the lead 2|, the loading of circuit 9 can be diminished by opening the switch. With the switch 29 open, the selectivity at the detector M is greatly increased. This is a desirable characteristic, since when the AFC is rendered inoperative the receiver should be as selective as possible. When the receiver is of the multi-range type, the arrangement is of particular value. The circuits l, 3 and 6 are shown provided With band changing devices I'--- The dotted line 6|, between correlated so that switch 20 will be opened alternately with adjustments to the taps of devices l'3'6'. Also, any desired relation between the two devices may be used. In this way there is provided a mechanism for rendering the AFC action inoperative while simultaneously improving the selectivity to the audio detector; further, the mechanism is employed, where the receiver is of the multi-band type, to cutout the AFC 7 action with selectivity improvement on those bands where the AFC is notdesired. I

While I have indicated and described one system forcarrying my invention-into effect, it -will be apparent to one skilled in the art that my invention is byno means limitedto the particular organization shown and described, but that many modifications may be madelwithout departing from the scope ofmy invention,,as set forth in the appended claims. 1

What I claim is: 1

1. In combination with at. least two cascaded coupled resonant circuits tuned to the same carrier frequency, a source'of carrier wav es coupled to the first of. the circuits, a detector coupled to the second of the circuits, means coupled to the said first circuit for deriving av direct current voltage therefrom which is dependent in magnii tude and polarity upon the carrier frequency departure, said deriving means normally loading 7 said first circuit,.and switch means operatively associated with said first circuit and deriving means for removing the loading efiect'of the latter thereby to decrease-the said first circuit loading and increase the selectivity to said detector. A I

2. In combination with at least two cascaded coupled resonant circuits tuned to the same carrier frequency, a source of carrier wavesjcoupled to the first of the circuits, a detector coupled to the second of the circuits, means coupled to the said first ircuit for deriving a direct current voltage therefrom which is dependent in magnitude and polarity upon the carrier frequency departure, said deriving means normally loading said first circuit, and switch means operatively associated with said first circuit and deriving meansfor removing the loading effect of the-latter 1 thereby to decrease the said first circuit loading and increase the selectivity to said detectonand an oscillator network, electrically associated with said wave source, having a frequency adjusting connection tosaid deriving means.

3. In combination with atleast two cascaded coupled resonant circuits tuned to the same carrier frequency, a source of carrier waves coupled to the first of the circuits, a detector coupled to the second of the circuits, means coupled to-the said first circuit for deriving a direct current voltage therefrom which is dependent in magnitude and polarity upon the carrier frequency departure, and means for rendering said deriving means ineffective thereby to decrease the said first circuit loading and increase the selectivity to said detector, said deriving means comprising opposed rectifiers, and said last means including a switch connected in the common return path from said first circuit to the two rectifiers.

4. In a superheterodyne receiver, an intermediate frequency transmission network having a resonant output circuit, a second resonant circuit, tuned to the intermediate frequency, coupled to the first circuit, a pair of opposed rectifiers with a common resistive load coupled to the second circuit and normally loading it, a third resonant circuit, tuned to said frequency, coupled to the second circuit, a demodulator coupled to the third circuit, and switch means operatively associated with the second circuit and rectifiers for reducing the loading effect of said opposed rectifiers and resistive load thereby to increase the selectivity at the demodulator.

5. In a superheterodyne receiver, an intermediate frequency transmission network having a resonant output circuit, a second resonant circuit, tuned to the intermediate frequency, coupled to the first circuit, a pair of opposed rectifiers with a common resistive load coupled to the second circuit and normally loading it, a

third resonant circuit, tuned to said frequency,

coupled to the second circuit, a demodulator coupled to the third circuit, and means for reducing the loading effect of, said opposed rectifiers and resistive load thereby to increase the selectivity at the demodulator, said reducing means including a switch device connected between the midpoint of the second circuit and the midpoint of the resistive load.

to the third circuit, and adjustable means solely operatively associated with the second circuit and said rectifiers for reducing the loading'effect of said opposed rectifiers and resistive load thereby to increase the selectivity at the demodulator, a local oscillator having'a tunable tank network, a frequency control tube connected to the tank network and having a connection to the resistive load.

'7. In a multi-wave band receiver, signal and oscillator networks, means for selecting the wave band for reception by the receiver, 'a discriminator having a beat frequency input circuit for producing a voltage dependent solely upon a re-- ceived carrier frequency shift, a demodulator having a beat frequency input circuit coupled in cascade with said discriminator input circuit, means utilizing said voltage to adjust the oscillator frequency, and means, conjointly operative with the band selecting means, for simultaneously impairing the operation of the discriminator and increasing the selectivity to the demodulator when selecting a wave band over which the oscillator adjusting means is to be ineffective.

8. In a multi-wave band receiver, signal and oscillator networks, means for selecting the Wave band for reception by the receiver, a discriminator having a beat frequency input circuit for'producing a voltage dependent solely upon a received carrier frequency shift, a demodulator having a beat frequency input circuit coupled in cascade with said discriminator input circuit, means utilizing said voltage to adjust the oscillator frequency, and additional means, conjointly operative with the band selecting means, for simultaneously impairing the operation of the discriminator and increasing the selectivity to the demodulator when selecting a wave band over GARRARD MOUNTJOY. 

